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The ideal spectral averaging method depends on one’s science goals and the available information about one’s data. Including low-quality data in the average can decrease the signal-to-noise ratio (S/N), which may necessitate an optimization method or a consideration of different weighting schemes. Here, we explore a variety of spectral averaging methods. We investigate the use of three weighting schemes during averaging: weighting by the signal divided by the variance (“intensity-noise weighting”), weighting by the inverse of the variance (“noise weighting”), and uniform weighting. Whereas for intensity-noise weighting the S/N is maximized when all spectra are averaged, for noise and uniform weighting we find that averaging the 35%–45% of spectra with the highest S/N results in the highest S/N average spectrum. With this intensity cutoff, the average spectrum with noise or uniform weighting has ∼95% of the intensity of the spectrum created from intensity-noise weighting. We apply our spectral averaging methods to GBT Diffuse Ionized Gas hydrogen radio recombination line data to determine the ionic abundance ratio,y⁺, and discuss future applications of the methodology.

 

The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4 to 8 GHz. The nominal survey zone is 32.°3 >ℓ> −5°, ∣b∣ < 0.°5. Here, we analyze GDIGS Hnαionized gas emission toward discrete sources. Using GDIGS data, we identify the velocity of 35 Hiiregions that have multiple detected RRL velocity components. We identify and characterize RRL emission from 88 Hiiregions that previously lacked measured ionized gas velocities. We also identify and characterize RRL emission from eight locations that appear to be previously unidentified Hiiregions and 30 locations of RRL emission that do not appear to be Hiiregions based on their lack of mid-infrared emission. This latter group may be a compact component of the Galactic Diffuse Ionized Gas. There are an additional 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane. We compare these objects’ RRL data to¹³CO, Hi,and mid-infrared data, and find that these sources do not have the expected 24μm emission characteristic of Hiiregions. Based on this comparison we do not think these objects are Hiiregions, but we are unable to classify them as a known type of object.

 

Standard stellar evolution models that only consider convection as a physical process to mix material inside of stars predict the production of significant amounts of³He in low-mass stars (M< 2M_⊙), with peak abundances of³He/H ∼ few × 10⁻³by number. Over the lifetime of the Galaxy, this ought to produce³He/H abundances that diminish with increasing Galactocentric radius. Observations of³He⁺in Hiiregions throughout the Galactic disk, however, reveal very little variation in the³He abundance with values of³He/H similar to the primordial abundance,${\left({}^3\mathrm{H}\mathrm{e}/\mathrm{H}\right)}_p\sim {10}^{-5}$. This discrepancy, known as the “³He problem,” can be resolved by invoking in stellar evolution models an extra mixing mechanism due to the thermohaline instability. Here we observe³He⁺in the planetary nebula (PN) J320 (G190.3–17.7) with the Jansky Very Large Array to confirm a previous³He⁺detection made with the Very Large Array that supports standard stellar yields. This measurement alone indicates that not all stars undergo extra mixing. Our more sensitive observations do not detect³He⁺emission from J320 with an rms noise of 58.8μJy beam⁻¹after smoothing the data to a velocity resolution of 11.4 km s⁻¹. We estimate an abundance limit of³He/H ≤ 2.75 × 10⁻³by number using the numerical radiative transfer code NEBULA. This result nullifies the last significant detection of³He⁺in a PN and allows for the possibility that all stars undergo extra mixing processes.

 

The Outer Scutum-Centaurus spiral arm (OSC) is the outermost molecular spiral arm in the Galaxy and contains the most distant known high-mass star formation regions in the Milky Way. HII regions are the archetypical tracers of high-mass star formation, and because of their high luminosities, they can be seen across the entire Galactic disk from mid-infrared to radio wavelengths. We have detected HII regions at nearly 20 locations in the OSC, as far as 23.5 kpc from the Sun and 15 kpc from the Galactic center on the far side of the Galactic center. The far outer Galaxy has lower metallicity than the more inner regions of the Milky Way, with 12 + log(O/H) = 8.29 at the OSC versus 8.9 and 8.54 at the Galactic Center and the Solar neighborhood, respectively. Coupled with lower gas densities, star formation in the OSC could be similar to that of a much younger Milky Way or galaxies like the Large Magellanic Cloud. We find large reservoirs of diffuse and dense molecular gas (13CO, HCO+, HCN) in the OSC with the Argus array on the Green Bank Telescope (up to 105 Solar masses). We are also able to estimate the central ionizing sources from Very Large Array continuum observations, showing central stellar types as early as O4. Combined, these observations allow us to study chemical abundances and star formation efficiencies on the outer edge of the Milky Way, putting constraints on star formation properties towards the edge of the Galaxy’s molecular disk. 

The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4–8 GHz. The nominal survey zone is 32.3° > l > -5°, |b| < 0.5°. Here, we analyze GDIGS Hnα ionized gas emission toward discrete sources with sizes comparable to the 2.065' GDIGS Hnα beam. We use GDIGS data to identify the correct velocity of 39 H II regions that have multiple RRL velocity components. We identify and characterize RRL emission from 88 H II regions that previously lacked measured ionized gas velocities. We additionally identify and characterize RRL emission from eight locations that appear to be previously-unidentified H II regions and 41 locations of RRL emission that do not appear to be H II regions based on their lack of mid-infrared emission. We identify 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane and we compare the objects’ RRL data to 13CO, H I and mid-infrared data. These sources do not have the expected 24 μm emission characteristic of H II regions. Based on this comparison we do not think these objects are H II regions, but we are unable to classify them as a known type of object. 

Kinematic distance determinations are complicated by a kinematic distance ambiguity (KDA) within the Solar orbit. For an axisymmetric Galactic rotation model, two distances, a "near" and "far" distance, have the same radial velocity. Formaldehyde (H2CO) absorption measurements have been used to resolve the KDA toward Galactic HII regions. This method relies on the detection of H2CO absorption against the broadband radio continuum emission from HII regions. H2CO absorption at velocities between the HII region velocity and the maximum velocity along the line of sight (the tangent point velocity) implies that the HII region lies at the far kinematic distance whereas a lack of absorption implies that it lies at the near kinematic distance. The reliability of KDA resolutions using H2CO is unclear, however, as disagreements between distances derived using H2CO absorption and those derived using other methods are common. Here we use new H2CO and radio recombination line data from the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) to test whether H2CO absorption measurements can accurately resolve the KDA for 44 Galactic HII regions that have known distances from maser parallax measurements. For each of the 44 HII regions we determine whether the parallax distance is consistent with either the near or the far kinematic distance. We find that the Galactic distribution of H2CO is too sparse to reliably determine whether an HII region is at its near kinematic distance. The H2CO method also incorrectly resolves the KDA for 80% of HII regions that it places at the far kinematic distance; in such cases H2CO absorption may be caused by other sources of radio continuum emission (possibly the CMB, diffuse free-free, or synchrotron). Our results indicate that the H2CO method is unsuitable to resolve the KDA toward Galactic HII regions. 

Abstract We investigate the kinematic properties of Galactic H ii regions using radio recombination line (RRL) emission detected by the Australia Telescope Compact Array at 4–10 GHz and the Jansky Very Large Array at 8–10 GHz. Our H ii region sample consists of 425 independent observations of 374 nebulae that are relatively well isolated from other, potentially confusing sources and have a single RRL component with a high signal-to-noise ratio. We perform Gaussian fits to the RRL emission in position-position–velocity data cubes and discover velocity gradients in 178 (42%) of the nebulae with magnitudes between 5 and 200 m s − 1 arcsec − 1 . About 15% of the sources also have an RRL width spatial distribution that peaks toward the center of the nebula. The velocity gradient position angles appear to be random on the sky with no favored orientation with respect to the Galactic plane. We craft H ii region simulations that include bipolar outflows or solid body rotational motions to explain the observed velocity gradients. The simulations favor solid body rotation since, unlike the bipolar outflow kinematic models, they are able to produce both the large, >40 m s − 1 arcsec − 1 , velocity gradients and also the RRL width structure that we observe in some sources. The bipolar outflow model, however, cannot be ruled out as a possible explanation for the observed velocity gradients for many sources in our sample. We nevertheless suggest that most H ii region complexes are rotating and may have inherited angular momentum from their parent molecular clouds.